1. Field of the Invention
The present invention relates to the purification of crude metal alkyls containing solid particulate contaminants and, more particularly, to the clarification of aluminum alkyls containing such contaminants.
2. Description of the Prior Art
Fatty alcohols find widespread industrial use as, for example, in the manufacture of surfactants. While these alcohols can be derived from natural sources such as animal fats and vegetable oils, they can also be produced synthetically via Ziegler chemistry. In the Ziegler process, an olefin (usually ethylene) is polymerized onto a trialkyl aluminum such as triethyl aluminum in what is known as a "growth reaction" to produce higher straight chain aluminum alkyls. These alkyls are then oxidized to produce the corresponding alkoxide, which, upon hydrolysis, forms the corresponding alcohol.
Most commercial alcohol processes utilizing Ziegler chemistry utilize a growth reaction, e.g., adding ethylene to aluminum diethyl hydride to manufacture normal primary alcohols with an even number of carbon atoms in each chain. The nature of the materials handled throughout the process creates many problems. For example, in the case of aluminum alkyls, dense slurries of aluminum and aluminum triethyl (ATE) and solvent must be handled in pipelines and pumped at high pressures. ATE is pyrophoric and decomposes explosively in the presence of water and, accordingly, can be very dangerous if not handled properly. Although ATE can be diluted with inert solvent, making handling somewhat less dangerous, care must still be taken to prevent spills or leaks in any plant stream that contains ATE.
Typically, crude aluminum alkyls, e.g., ATEs, are produced from aluminum powder, hydrogen, and ethylene. The crude product consists of aluminum alkyls, most of which are aluminum triethyl, a solvent diluent such as kerosene or another inert hydrocarbon, and a finite amount of solid contaminants such as fine aluminum powder. These solid contaminants must be removed in order to obtain a product suitable for subsequent uses such as in a growth reaction wherein higher carbon chain aluminum alkyls are produced. The presence of these solid contaminants in the crude aluminum trialkyl (ATA) stream causes it to have a very dark or black appearance. Generally speaking, solid impurities including the aluminum fines are present in the crude ATA in an amount of up to about 15% by weight.
Numerous techniques have been employed in an attempt to commercially remove solid impurities from ATA streams. U.S. Pat. No. 2,863,894 teaches the filtration of aluminum alkyls. U.S. Pat. No. 2,900,402 shows that aluminum diethyl hydride present after a hydrogenation reaction can be filtered or decantered to remove aluminum fines. U.S. Pat. No. 2,931,820 teaches purification of an aluminum alkyl by filtration. U.S. Pat. No. 2,987,534 teaches the removal of suspended aluminum by centrifuging. U.S. Pat. No. 3,030,401 teaches the removal of aluminum powder by filtering the reaction solution in the presence of a filter aid such as bentonite clays. U.S. Pat. No. 3,032,574 teaches the removal of suspended aluminum impurities by centrifuging, vacuum distillation, or filtration. U.S. Pat. No. 3,050,540 teaches the removal of excess aluminum from aluminum alkyls by filtration. U.S. Pat. No. 3,076,006 teaches that suspended solids can be removed by filtration. U.S. Pat. Nos. 3,207,770; 3,207,772; 3,207,773; and 3,207,774 all disclose that suspended aluminum in aluminum trialkyl products can be removed by distillation, centrifuging, or filtration.
None of the filtration methods described heretofore have proven to be commercially viable for removing the solid impurities, including the aluminum fines, from ATA streams. Indeed, the use of prior art filtration methods have proven to be highly dangerous and unreliable due to constant plugging of the filters, the need for replacement, and cracking under high pressures, resulting in aluminum alkyl spills, which, as noted above, can be extremely hazardous, both to personnel and equipment because of the pyrophoric nature of the aluminum alkyls. Accordingly, the use of filters as a means of removing solid impurities from ATA streams has been largely discontinued in commercial operations and replaced by vacuum distillation.
While vacuum distillation can successfully remove the solid impurities from the ATA streams, such processes are inefficient both in energy consumption and lost product in that some of the ATAs, e.g., aluminum trioctyl, will decompose at the temperature needed to conduct the distillation. It is clear that filtration, if it could be conducted safely, continuously, and over long run times, would be the preferred method of purifying ATA streams.